The present invention relates to a network interface module suitably used as a cable television cable modem and a set-top box.
Cable television companies are building arterial networks in increasing numbers, using optical fiber or HFC (hybrid fiber/coax) to offer multi-channel and wide band data communication service by way of empty channels, even when a coaxial cable is used for the lead-in line into individual homes. Consequently, by using 64-value QAM (Quadruture Amplitude Modulation), a high speed data line can be established such that a single channel has a bandwidth of 6 MHz and the transmission rate is 30 Mbps. In the above-noted cable modem or set-top box, a network interface module is mounted for establishing a connection to such a cable network.
FIG. 3 is a block diagram showing an electrical arrangement of a typical conventional set-top box 1. A downstream signal ranging, for example, from 54 to 860 MHz is supplied for input to an input terminal 2 from a cable line, and an upstream signal ranging, for example, from 5 to 42 MHz is supplied for output to the cable line from the input terminal 2.
In recent years, the downstream signal in cable television broadcasting is constituted by analogue NTSC visual and audio signals, digital QAM visual and audio signals, and a QPSK (Quadruture Phase Shift Keying Modulation) data signal. Therefore, the downstream signal, having been filtered by a high pass filter 3 that is an intermediate frequency filter having 5 to 46 MHz as an attenuation band and 54 MHz or higher as a passing band, is first separated by a directional coupler 4, then supplied for input to a digital tuner 5, and supplied to an analogue tuner 7 and a QPSK tuner 8 via a half-separator 6.
The tuners 5, 7, and 8 are each constituted by a receiver circuit for a UHF band covering 470 to 860 MHz (B3 band), another one for VHF High band covering 170 to 470 MHz (B2 band), and another one for VHF Low band covering 54 to 170 MHz (B1 band). However, there is no particular specification for band separation.
An intermediate frequency signal of digital television broadcasting of the desired channel selected through the digital tuner 5, having gone through a digital SAW filter 9, is supplied to a down converter 10 to be converted to an intermediate frequency signal of a low frequency, and converted to an 8- or 10-bit digital signal by an analogue/digital converter 11. The digital signal is I,Q-demodulated by a QAM demodulator 12, error-corrected, and supplied for output to a transport decoder 13 as a serial bit stream. The transport decoder 13 extracts visual and audio signals from the serial bit stream and supplies the extracted signals to an MPEG2 decoder 14. The MPEG2 decoder 14 decodes the bandwidth of the incoming visual and audio signals, and sends out the decoded signals to a multiplexer 15 as analogue NTSC composite visual and audio signals.
Meanwhile, an intermediate frequency signal of analogue television broadcasting of the desired channel selected through the analogue tuner 7 is demodulated into analogue NTSC composite visual and audio signals by an NTSC demodulator 16. The composite visual and audio signals from the MPEG2 decoder 14 and NTSC demodulator 16 are selected by the multiplexer 15, converted into a television signal of a desired channel such as channel 1, 2, or 13 by a high frequency modulator 17, and supplied for output from an output terminal 18 to an antenna input terminal of a television set.
A data signal selected through the QPSK tuner 8 is demodulated by the a QPSK demodulator 19 and supplied to a processor circuit 20 realised by a microprocessor and other components. Upstream data such as information on viewing fees, incidentally, is supplied for input to a QPSK modulator 21 from the processor circuit 20 to be modulated, amplified by a power amplifier 22, thereafter led in to a data terminal 23, and led out to the input terminal 2 via a low pass filter 24 that is an upstream circuit.
In the conventional set-top box 1 arranged in the above manner, a network interface module is structured from the high pass filter 3, the low pass filter 24, the directional coupler 4, the half-separator 6, the tuners 5, 7, and 8, the digital SAW filter 9, the down converter 10, the analogue/digital converter 11, the QAM demodulator 12, the NTSC demodulator 16, and the QPSK demodulator 19.
Therefore, in such a network interface module, digital noise and high frequency noise are produced in high frequency circuits including the tuners 5, 7, and 8, the high pass filter 3, the low pass filter 24, the directional coupler 4, and the half-separator 6 by the processor circuit 20, the QPSK modulator 21, the QPSK demodulator 19, the QAM demodulator 12, etc., and degrade properties, such as C/N, BER (Bit Error Rate) and phase noise, representing digital demodulation performance. This is problematic in designing a set-top box.
For these reasons, as a conventional counter-measure to solve the noise problem, the high pass filter 3, the low pass filter 24, the digital tuner 5, the analogue tuner 7, QPSK tuner 8, and the high frequency modulator 17 are built in respective individual shielded cases so as to be handled as separate high frequency components. Consequently, when assembled into a set-top box, those high frequency components need to be assembled, making a complex arrangement inevitable and rendering the assembled set-top box expensive, bulky, and power-consuming.
The present invention has an object to offer a network interface module capable of providing a simpler arrangement and improvements on cost, size and power consumption.
The network interface module in accordance with the present invention is a network interface module for receiving cable television broadcasting, and, to achieve the above object, is characterised in that it comprises:
a low pass filter for transmitting an upstream signal to a cable television station;
a high pass filter for blocking the upstream signal and receiving a downstream signal from the cable television station; and
a directional coupler for supplying the downstream signal to a visual and audio signal receiver circuit and a control signal receiver circuit,
wherein the low pass filter, the high pass filter, and the directional coupler form a high frequency circuit built in a shielded package integrally as a module.
With the above-noted arrangement, since the high frequency circuit in the input stage is built in a shielded package integrally as a module, it becomes possible to reduce negative effects of noise on the high frequency circuit, the noise being produced by those circuits handling digital signals such as digital visual and audio signals and upstream and downstream data signals, and to improve C/N, BER, and other properties. It also becomes possible to build digital circuits in a shielded housing of the network interface module as a module, to simplify the circuit arrangement, and to reduce cost, size and power consumption.
Further to the above arrangement, the visual and audio signal receiver circuit is preferably a tuner that is capable of both analogue and digital signals.
With the above-noted arrangement, it becomes possible to omit the arrangement for separation, to eliminate insertion loss caused by separator means. It also becomes possible to save space, to facilitate fabrication as a module, and to greatly reduce cost, size and power consumption.
In addition, the high frequency circuit further preferably includes a high frequency relay for a through output lead-out, the high frequency relay being provided to a stage prior to the visual and audio signal receiver circuit.
With the above-noted arrangement, since the high frequency relay does not consume as much space as an electronic switching circuit in enhancing applicability by providing a through output circuit, it becomes possible to facilitate fabrication as a module with other circuits, and to further reduce loss than a separator.